Silylated polyurethane resins are useful as coatings, adhesives, sealants and industrial elastomeric goods and can be prepared from polyols and isocyanatosilanes. Silylated polyurethane resins can be prepared by known methods. One such method is disclosed by Johnston, U.S. Pat. No. 5,990,257, wherein the resin is prepared by the reaction of a polyol and a diisocyanate to form hydroxyl terminated prepolymer followed by the addition of an isocyanatosilane. Another suitable method for the preparation of silylated polyurethane resin is taught by Huang in published U.S. patent application Ser. No. 11/260,598, by which the isocyanatosilane may be added to the reaction mixture of polyol and diisocyanate before all of the diisocyanate has been reacted. Resins prepared by these and other methods may contain small amounts of residual isocyanate, whether inadvertently or by design.
When preparing silylated polyurethane resins, typically there is a hydroxyl functional group for every isocyanate group so that at the completion of the reaction there should be no residual isocyanate. In reality small amounts of isocyanate and hydroxyl groups remain when the manufacturing process is complete because the reactions proceed to completion very slowly due to the low levels of reactants. Thus, the small amount of residual isocyanate slowly reacts over time with the residual hydroxyl-terminated polymer resulting in an undesirable increase in product viscosity. Alternatively, to ensure that all of the isocyanate has reacted the reaction mixture can be held under reaction conditions for a prolonged period of time, during which the viscosity may increase to an undesirable level and the viscosity may vary significantly from batch-to-batch. During the production of silylated polyurethane resins it is difficult to ensure that all of the isocyanate has reacted with the hydroxyl-terminated polymer.
Resins free of isocyanates are desirable because isocyanates are toxic and present health risks, and because even small levels of residual isocyanate in the resin can cause viscosity instability, i.e. the viscosity of the resin can increase on storage after manufacture due to the slow reaction of the residual isocyanate. Efforts to “react out,” or remove all of the isocyanate can result in resins that have viscosity variability from batch-to-batch.
There remains a need in the industry for a process to manufacture silylated polyurethane resin that is free of residual isocyanate and provides consistent viscosity and viscosity stability.